It’s a fairly speedy clear, with progression races such as this usually taking a little longer to complete - generally one later boss will hold off the majority of top guilds for at least a couple of weeks as they continue to build up gear. In Legion, gear is a little easier to come by due to the addition of the Mythic+ dungeon difficulty that rewards high-level loot on a more repeatable basis than previous, plus the addition of multiple Legendary items that push player power above the norm and the Titanforging mechanic that allows loot to randomly be of much higher power than normal.
That said, it’s not like Blizzard weren’t aware of those changes going in, and they could have tuned encounters accordingly. It doesn’t actually affect the other 99.999996% of the player base that’s unlikely to see Mythic Xavius for a good couple of months, but tracking the best players and how long it takes them to ‘beat’ a new expansion is something the community enjoys getting involved with. There’s also the natural fantasy of being first, quickly dispelled when things are cleared less than a day after release.
For those interested in seeing the later encounters, kill videos will probably start coming out over the rest of the week. I recommend the PrestigeGaming stream (which should kick off at about 1pm CEST / Midday BST / 7am EDT / 4am PDT today) as the furthest advanced live coverage there is, stopping yesterday after a huge number of attempts at the fifth boss, Il’gynoth. There are also now kill videos for the second boss on WoWProgress, including this rather nice Demon Hunter one: